Tomato diseases and disorders by Mark L. Gleason and Brooke A. Edmunds, Department of Plant Pathology Tomatoes are the most popular vegetable crop in Iowa. Many diseases and disorders can affect tomatoes during the growing season. This publication describes the symptoms and management of common problems found in gardens and greenhouses.
Figure 1. Septoria leaf spot symptoms
Diseases in outdoor production This section looks at the diseases common in outdoor gardens. Septoria leaf spot Septoria leaf spot, caused by the fungus Septoria lycopersici, is the most common foliar disease of tomatoes in Iowa. It first appears as small, water-soaked spots that soon become circular spots about 1â „8 inch in diameter (Figure 1). The lesions gradually develop grayish white centers with dark edges (Figure 2). The light-colored centers of these spots are the most distinctive symptom of Septoria leaf spot. When conditions are favorable, fungal fruiting bodies appear as tiny black specks in the centers of the spots. Spores are spread to new leaves by splashing rain. Heavily infected leaves turn yellow, wither, and eventually fall off. Lower leaves are infected first, and the disease progresses upward if rainy weather persists. Defoliation can be severe after periods of prolonged warm, wet weather (Figure 3). Infection can occur at any stage
Figure 2. Septoria leaf spot; the light colored centers distinguish them from leaf spots caused by bacterial spot and speck
of plant development but appears most frequently after plants have begun to set fruit. The fungus survives the winter in tomato debris. To control Septoria leaf spot a combination of cultural practices is often needed. These practices, which also will help to reduce the risk of many other diseases, include the following: 䥲 Plant disease-free transplants far enough apart that the plants will not be crowded after they are full grown, in order to help the foliage dry rapidly.
Tomato diseases and disorders | Diseases in outdoor production
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䡲 Water at the base of the plants, and in the morning rather than the evening, to minimize the amount of time that the leaves are wet. 䡲 Remove as much plant debris as possible in the fall and promptly plow under or bury the remaining residue.
Cultural and chemical controls for early blight are the same as for Septoria leaf spot. In addition, avoid potato in rotations, and harvest all ripe fruit at every picking to avoid infecting other fruit. Resistant varieties of tomatoes in the Mountain series (Mountain Supreme, Pride, Gold, Fresh, and Belle) provide partial resistance to early blight.
䡲 Rotate crops so that tomatoes are grown in the same ground only every three or four years. 䡲 Avoid working with plants when foliage is wet to avoid spreading disease-causing microorganisms. Fungicides also can help to control Septoria leaf spot. Consult ISU Extension bulletin Insects and Diseases in the Home Vegetable Garden (PM 230) for current fungicide recommendations. Early blight Early blight, caused by the fungus Alternaria solani, is also known as Alternaria leaf spot or target spot. Like Septoria leaf spot, early blight is common in Iowa tomato plantings, and the two diseases may attack the same plants. Premature loss of lower leaves is the most obvious symptom of the disease (Figure 3). Brown to black spots, 1 ⁄4 to 1⁄2 inch in diameter with dark edges, appear on lower leaves (Figure 4). Spots frequently merge, forming irregular blotches. Dark, concentric rings often appear in leaf spots, resulting in the “target” appearance suggested by the common name. Leaves turn yellow and dry up when only a few spots are present. The fungus occasionally attacks fruit at the stem end, causing large, sunken areas with concentric rings and a black, velvety appearance (Figure 5). Warm, wet weather favors rapid spread of early blight. A. solani also can infect potato. Like Septoria leaf spot, early blight can infect plants at any stage during the growing season but usually progresses most rapidly after plants have set fruit.
Figure 3. Defoliation caused by both Septoria leaf spot and early blight
Figure 4. Early blight rot on foliage
Figure 5. Early blight symptoms on fruit
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Tomato diseases and disorders | Diseases in outdoor production
Anthracnose Affected plants die early and produce few, if any, fruit. Anthracnose, caused by the fungus Colletotrichum Splitting open an infected stem reveals brownish streaks coccodes, is probably the most common fruit-attacking extending up and down the stem (Figure 8). These disease of tomato in Iowa. Symptoms first become visible discolored streaks are the water-conducting tissue, which on ripe or ripening fruit as small, circular, indented spots becomes plugged during attack by the fungus, leading to in the skin. As these spots expand, they develop dark wilting of the leaves. Plants are susceptible at all stages of centers or concentric rings of dark specks, which are the development, but symptoms are most obvious at or soon spore-producing bodies of the fungus (Figure 6). In moist after flowering. weather these bodies exude large numbers of spores, giving diseased areas a cream to salmonpink color. By this stage, decay has penetrated deeply into the tomato flesh. Spotted fruits often may rot completely because of attack by secondary fungi through anthracnose spots. Anthracnose appears most Figure 6. Anthracnose fruit rot commonly on overripe fruits. The fungus survives the winter on diseased tomato vines, in the soil, and in seeds. Weeks before the fruit ripens, anthracnose can become established on leaf spots caused by other fungi or by insect feeding injuries. Spores are spread largely by rain splash. Warm, wet weather causes the disease to spread and symptoms to develop. While insect or other wounds facilitate infection, tomatoes can also become infected in the absence of wounds. Control measures for anthracnose are the same as for Septoria leaf spot. In addition, harvest at frequent intervals and pick all ripe fruit at each harvest. Fusarium wilt Fusarium oxysporum f. sp. lycopersici, the fungus that causes Fusarium wilt, attacks only certain tomato cultivars. Plants infected by this soil-dwelling fungus show leaf yellowing and wilting that progress upward from the base of the stem. Initially, only one side of a leaf midrib, one branch, or one side of a plant will be affected. The symptoms soon spread to the remainder of the plant (Figure 7). Wilted leaves usually drop prematurely.
To minimize losses from Fusarium wilt, Figure 7. Fusarium wilt it is advisable to plant resistant varieties, and many resistant varieties are available. The letter “F� following the variety name indicates resistance to one or more races of the Fusarium fungus. Resistant varieties may become infected, but Figure 8. Vascular browning caused by Fusarium wilt disease will not be as severe as with susceptible varieties and a reasonable yield should still be obtained. In addition, plant disease-free seed or transplants in welldrained, disease-free soil, rotate at least four years away from tomatoes to reduce populations of the fungus in soil, and remove and destroy infected plant residue. In greenhouse or seedbeds, disinfest soil by treating with steam.
Tomato diseases and disorders | Diseases in outdoor production
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Verticillium wilt Verticillium albo-atrum and Verticillium dahliae, the fungi that cause Verticillium wilt, can attack more than 200 plant species, including potato, pepper, eggplant, strawberry, watermelon, and radish. Like Fusarium wilt, this disease appears first on the lower leaves and progresses upward. Yellow blotches develop on lower leaves; the leaves rapidly turn completely yellow, wither, and drop off (Figure 9). Unlike Fusarium wilt, symptoms of Verticillium wilt do not progress along one side of a leaflet, branch, or plant. Infected plants may survive through the growing season, but are stunted and yield is reduced. Verticillium wilt, like Fusarium wilt, causes internal browning of the water-conducting tissue in stems (Figure 9). The discoloration is most pronounced near the soil line and seldom extends more than 10 to 12 inches above this point. Control measures are similar to those for Fusarium wilt. Names of Verticillium-resistant tomato cultivars are followed by the letter “V.� Rotate away from all crops in the tomato family (Solanaceae), including tomato, pepper, potato, and eggplant, for at least four years. Corn and beans are suitable rotation crops in the home garden.
Late blight Late blight, caused by the fungus Phytophthora infestans, rarely occurs in Iowa but can devastate tomato plantings during periods of cool, rainy weather. Late blight may infect either young (upper) or old (lower) leaves. It first appears as water-soaked areas that enlarge rapidly, forming irregular, greenish black blotches (Figure 10), giving the plant a frost-damaged appearance. The undersides of the leaves often show a downy white growth in moist weather. Infection of green or ripe fruit produces large, irregularly shaped brown blotches (Figure 11). Infected fruits rapidly deteriorate into foul-smelling masses. Late blight usually appears in mid- or late August during persistent cool, wet weather, or when cool night temperatures cause frequent heavy dews. P. infestans causes similar symptoms on potatoes and can spread from potatoes to tomatoes. Control measures for late blight are the same as for Septoria leaf spot. In addition, avoid rotating with potato.
Figure 9. Verticillium wilt foliage symptoms and cut stem showing browning
Figure 10. Late blight symptoms on leaflet
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Figure 11. Fruit rot caused by late blight
Tomato diseases and disorders | Diseases in outdoor production
Bacterial spot Bacterial spot, caused by the bacterium Xanthomonas campestris pv. vesicatoria, infects both tomato and pepper. Spots that appear on leaves and stems are small (up to 1 ⁄8 inch across), circular to irregular in shape, and have a slightly greasy feel. Unlike similar-sized spots caused by the fungus Septoria lycopersici, those caused by the bacterial spot pathogen do not develop grayish brown centers. As lesions enlarge, they often become surrounded by a yellow halo. If spots are numerous, they begin to grow together (Figure 12), and leaves wither and turn brown (Figure 13). Fruit symptoms are more distinctive than leaf or stem symptoms. Spots on green fruit first appear as black, raised, pimple-like dots surrounded by water-soaked areas. As the spots enlarge to 1⁄4 to 1⁄2 inch, they become gray-brown and scabby with sunken, pitted centers (Figure 14). The bacterium overwinters on the surface of seeds, in infected debris, and in soil. It is commonly brought into fields on infected transplants. Warm, rainy weather favors rapid spread of bacterial spot. Control measures are essentially the same as for Septoria leaf spot. However, obtaining disease-free transplants is particularly crucial for controlling this and other bacterial diseases, since the bacteria can be transmitted to seedlings from contaminated seeds. Avoid rotating with
peppers. It is advisable to avoid handling plants (pruning and tying, for example) any more than is necessary, because wounds caused by handling allow bacteria to enter plants. Sprays of a fixed copper product can reduce spread of the disease in the garden if applications begin when first symptoms appear. Refer to Insects and Diseases in the Home Vegetable Garden (PM 230) for current spray recommendations. Bacterial speck This disease, caused by the bacterium Pseudomonas syringae pv. tomato, does not affect pepper or other solanaceous crops but may survive on nonhost plants. Tiny, 1⁄16-inch-diameter, dark spots appear on leaves, surrounded by yellow halos (Figure 15). However, as with bacterial spot and bacterial canker, the fruit symptoms are most characteristic. The numerous specks that develop on young green fruit are slightly raised, 1⁄32 to 1⁄16 inch in diameter, and have well-defined margins (Figure 14). The specks are considerably smaller than the spots caused by bacterial spot, do not penetrate the fruit deeply, and can be scraped off with a fingernail. Although bacterial speck seldom reduces yields greatly, it can harm fruit quality. Infection is favored by cool (less than 70° F), wet conditions. Epidemics often follow rainstorms that cause abrasion of leaves and splash soil onto the foliage.
Figure 14. Fruit spots caused by bacterial speck (left) and bacterial spot (right)
Figure 12. Bacterial spot on leaflet
Figure 13. Wilting caused by bacterial spot
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P. syringae pv. tomato overwinters in seed, in plant debris, in soil, and on many other plants. To control bacterial speck, follow the same cultural measures as for Septoria leaf spot. Plant only disease-free transplants. Destroying weeds around a tomato field or garden will help to reduce survival of the causal bacterium. A fixed copper product can help control spread of bacterial speck if applications begin when the first symptoms appear.
Figure 15. Bacterial speck on leaflet
Figure 16. Wilting of a young transplant (center) caused by bacterial canker
Figure 17. Marginal browning of leaves caused by bacterial canker
Bacterial canker Bacterial canker, caused by the bacterium Clavibacter michiganensis subsp. michiganensis, has caused serious losses in some tomato plantings in the North Central states, including Iowa, during the last few decades. Young transplants may wilt suddenly and completely (Figure 16). On older plants, leaflets begin to turn brown at the edges, then die back progressively toward the leaf midrib (Figure 17). Often only one side of a leaflet or a plant develops symptoms first, but symptoms eventually spread. Rarely, cavities may develop within stems, sometimes splitting open into brown, longitudinal cankers. Spots on fruit are quite distinctive: white and slightly raised at first, then raised, dark-colored centers with white halos 1⁄16 to 1⁄8 inch in diameter (Figure 18). These spots are sometimes termed “bird’s-eye” lesions. The white halo turns brown as the spot becomes older. Control measures for bacterial canker are the same as for bacterial speck, except that copper sprays have minimal impact on slowing the spread of bacterial canker.
Figure 18. Fruit spots caused by bacterial canker
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Tomato diseases and disorders | Diseases in outdoor production
Viruses The most common virus disease in Iowa is tomato spotted wilt virus (TSWV), but others can occur. TSWV causes distinctive yellow ringspots on mature fruit (Figure 19). Foliage also can be affected; plants are usually stunted and tip leaves show a purplish discoloration (Figure 20). Thrips, which are small (1⁄4 inch long) green-brown insects, spread the virus. Plants can be affected as transplants while growing in a greenhouse; after transplanting, stunting and failure to set fruit may be the most noticeable symptoms (Figure 21). Figure 19. TSWV ringspots on a fruit
Other viruses are spread by aphids or leafhoppers and can cause leaf curling, yellow or green mosaic patterns on the leaves, “shoestringing” of leaves, or a bronzing appearance. Fruits also are affected with mosaic patterns, streaking, or mottled areas.
Figure 20. Purplish discoloration of tip leaves caused by TSWV
Planting only certified virus-free transplants is the best technique for managing viruses. It is helpful to verify that the greenhouse used to produce transplants conducts a vigorous program to control aphids and thrips. There is no way to “cure” a virusinfected plant. However, removing the infected plant as soon as symptoms are found can help prevent spread by insects to healthy plants. Many viruses that infect tomato also infect peppers and potatoes; so, avoid planting these crops next to each other. Insect control may also be beneficial in the transplant and early-season phases. Consult Insects and Diseases in the Home Vegetable Garden (PM 230) for current insecticide recommendations.
Figure 21. Dwarfing of plant (left) by TSWV
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Diseases in greenhouse production High humidity and warm temperatures in greenhouses provide a favorable environment for development of certain diseases. The diseases listed below are much more likely to occur inside greenhouses than outdoors. Gray mold Gray mold, caused by the fungus Botrytis cinerea, is a common disease of greenhouse-grown tomatoes. This disease is characterized by a light-gray fuzzy growth that appears on stems and leaves. Soft rot of the stem end of the fruit can also occur (Figure 22). Botrytis infections are most severe in greenhouses with moderate temperatures, high humidity, and stagnant air. Increasing ventilation and air circulation to reduce humidity levels can be helpful, as well as timely fungicide applications. Leaf mold Leaf mold, caused by the fungus Fulvia fulva, can cause problems in humid greenhouses with poor air circulation. This fungal disease appears on lower leaves as yellow spots on the upper surface (Figure 23) and fuzzy masses of buff-colored spores on the underside (Figure 24). These leaves drop prematurely as the disease progresses upward on the plant. Lowering greenhouse humidity, planting resistant varieties, and applying fungicide promptly can be helpful in leaf mold management. Powdery mildew Powdery mildew, caused by the fungus Oidium neolycopersici, is also common in humid Iowa greenhouses with poor air movement. Characterized in the early stages by white patches on the upper surface of leaves, this disease can cause defoliation as the spots develop into brown lesions (Figure 25). Increasing air circulation and spacing between plants will reduce powdery mildew problems. Fungicide sprays also can be effective if used when symptoms are first noticed.
Figure 22. Gray mold on fruit
Figure 23. Yellow spots caused by leaf mold on upper leaf surface
Figure 24. Buff-colored spore masses of leaf mold on underside of leaflet
Good control of powdery mildew can be achieved by using several fungicides. Fungicides are most effective when sprays begin as soon as the first symptoms are noticed, rather than after the disease is already well established. Figure 25. Blighting of leaflet by powdery mildew
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Tomato diseases and disorders | Diseases in greenhouse production
Physiological disorders The following problems are not caused by infectious microorganisms but rather by environmental stresses on the plant. These disorders occur primarily in fieldgrown rather than greenhouse-grown tomatoes. Blossom end rot Blossom end rot is a very common problem on green and ripe tomatoes. It first appears as a sunken, brownish black spot 1â „2 to 1 inch in diameter on the blossom end of the fruit. These spots may gradually increase in size (Figure 26). Although blossom end rot itself causes only local injury, secondary organisms frequently invade Figure 26. Blossom end rot the lesion and cause complete rotting of the fruit. It often occurs in rapidly developing fruit during periods of hot, dry weather and tends to have the greatest impact on the earliestmaturing fruit. Blossom end rot is caused by a calcium deficiency that is related to wide fluctuations in available moisture. Iowa soils contain plenty of calcium, so the addition of calcium will not solve the problem. To prevent blossom end rot, maintain a steady rate of plant growth without stress. A consistent and ample supply of moisture can reduce the problem by helping to maintain a steady flow of calcium from the soil to the fruit. Mulching also will help by conserving soil moisture. Blossom end rot is more serious when an excess of nitrogen fertilizer has been applied. Staking and pruning tomato plants also may increase the
incidence of blossom end rot. If blossom end rot occurs, remove the affected fruit so that later-maturing fruit will develop normally. Mulching and avoiding heavy applications of nitrogen fertilizer may help reduce fruit cracking. Fruit cracking Two types of cracks may develop on tomato fruit. Radial growth cracks radiate from the stem (Figure 27), and concentric cracks encircle the fruit, usually on the shoulders (Figure 28). Similar to blossom end rot, cracking is associated with rapid fruit development and wide Figure 27. Radial fruit cracking fluctuations in water availability to the plant. Fruit that has reached the ripening stage during dry weather may show considerable cracking if the dry period is followed by heavy rains and high temperatures. Tomato varieties differ considerably Figure 28. Concentric fruit cracking in the amount and severity of cracking under climatic conditions. Supersonic and Jetstar are two varieties that show relatively low incidence of cracking. As with blossom end rot, mulching and avoiding heavy applications of nitrogen fertilizer should help reduce fruit cracking.
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Catfaced fruit Catfacing is a term used to describe misshapen fruit with irregular bulges at the blossom end and bands of leathery scar tissue (Figure 29). Cold weather at the time of blossom set distorts and kills certain cells that should develop into fruit, resulting in the deformities. The disorder is most often observed among first-formed fruit. Catfacing is most common in the large-fruited “beefsteak� type tomatoes. Sunscald Sunscald occurs on green tomato fruit exposed to the sun. The initial symptom is a whitish, shiny area that appears blistered. The killed, bleached tissues gradually collapse, forming a slightly sunken area that may become pale yellowish and wrinkled as the fruit ripens (Figure 30). The killed tissue is quickly invaded by secondary organisms and the fruit decays. Fruits most subject to sunscald are those that have been exposed suddenly to the sun because of pruning, natural spreading of the plant caused by a heavy fruit load, or loss of foliage from diseases. The extent of the injury is more serious during periods of abnormally high temperatures. To prevent sunscald on tomato fruit, control foliar diseases and avoid heavy pruning or shoot removal.
Figure 29. Catfaced fruit
Figure 30. Sunscald
Blotchy ripening This physiological disorder is indicated by the absence of normal red pigment on localized areas of the fruit. These areas appear as yellow or gray-green patches on otherwise normal-colored ripening fruit (Figure 31). When these fruits are sliced open, brown discoloration is often apparent. Figure 31. Blotchy ripening
Climatic, nutritional, and cultural problems may contribute to blotchy ripening. Low levels of potassium in plants and prolonged cloudy periods or inadequate light intensity have been associated with the disorder. Other possible contributing factors are high soil moisture, high humidity, low temperature, soil compaction, and excessive fertilization. These environmental factors can contribute to nutrient deficiencies or other
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imbalances that impede development of red pigment in the fruit. To minimize incidence of blotchy ripening, follow proper cultural practices to maintain nutritional balance and plant vigor. If commercial fertilizers are used, select balanced formulations and avoid over-application.
Tomato diseases and disorders | Physiological disorders
Physiological leafroll Physiological leafroll occurs when the edges of the leaves roll upward and inward (Figure 32). Sometimes the curling continues until the leaf margins from either side touch or overlap. Some leaves on the plant may not exhibit rolling. Leafroll does not reduce plant growth, yield, or fruit quality. It is Figure 32. Physiological leafroll believed to result from irregular water supply, and may be intensified following pruning. The symptoms are sometimes temporary, disappearing after a few days, but can persist throughout the growing season. Failure to set fruit High summer temperatures can reduce the number of tomato fruit harvested in several ways. High day and night temperatures will reduce flower production on tomato plants. If the night temperatures are above 70째 F, flower production and pollination are reduced. High temperatures for several consecutive days, coupled with drought conditions, will lead to poor pollination and cause flowers to drop from the plants. Hot drying winds may intensify the problem. Plants sometimes drop their flowers when night temperatures are lower than 55째 F. The most favorable night range for tomato fruit set is between 58 and 68째 F. Commercially available blossom-set hormones should not be relied upon because they do not give consistent results.
Herbicide injury This malady is caused by misapplication or drift of 2,4-D, MCPP, and other growth regulator herbicides. Tomato plants are highly sensitive to these chemicals throughout the growing season. The first symptom is downward curling of leaves and tips of growing points. Leaves often become narrow and twisted toward the tip, with prominent, light-colored veins. The symptoms are most pronounced on portions of the plant that were actively growing when the exposure occurred. In severe cases, stems and petioles become thick, stiff, and brittle with warty outgrowths (Figure 33). Affected plants usually recover. However, the fruit may become catfaced or develop in a plum shape, and may be hollow and seedless. To avoid herbicide injury, do not spray when wind may carry spray drift toward tomatoes or other sensitive crops. In addition, spray at low pressures, use a coarse-spray nozzle, and apply the spray as close to the ground as possible. Avoid applying other pesticides in sprayers that have previously contained herbicide because traces of herbicide are likely to remain in the sprayer even after thorough rinsing.
Figure 33. Herbicide damage
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For more information Contact your local Iowa State University Extension office for additional information or copies of the following publications. Canning and Freezing Tomatoes, PM 638 Container Vegetable Garden, PM 870B Garden Soil Management, PM 820 Organic Mulches for Gardens and Landscape Plantings, RG 209 Planting a Home Vegetable Garden, PM 819 Preserve Food Safely, N 3332 Questions about Composting, RG 206 Selling Fruits and Vegetables, PM 1887 Small Plot Vegetable Gardens, PM 870A Starting Garden Transplants at Home, PM 874 Watering the Home Garden—Use of Trickle Irrigation, PM 823 Where to Put Your Vegetable Garden, PM 814
Additional information also is available from these Web sites. ISU Extension publications http://www.extension.iastate.edu/store ISU Horticulture http://www.hort.iastate.edu/ Questions also may be directed to ISU Extension Hortline by calling 515-294-3108 during business hours (10 a.m.–12 noon, 1 p.m.–4:30 p.m. Monday–Friday). If you want to learn more about horticulture through training and volunteer work, ask your ISU Extension office for information about the ISU Extension Master Gardener program.
Prepared by Mark L. Gleason, extension plant pathologist, and Brooke A. Edmunds, graduate assistant, Department of Plant Pathology. Edited by Jean McGuire, extension communication specialist. Illustrated by Jane Lenahan, extension graphic designer. Designed by Donna Halloum, Creative Services, Instructional Technology Center, Iowa State University. Photography—Figure 2: Ron Pitblado, Ridgetown College, Ontario, Canada; Figure 4: University of Illinois; Figure 8, Clemson University; Figures 10 and 14: Alan MacNab, Penn State University; Figure 15: Jon Watterson; Figures 17 and 18: Richard X. Latin, Purdue University. File: Hort and LA 2-9
. . . and justice for all The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Jack M. Payne, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.
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Tomato diseases and disorders | For more information